Interpretation of interim F-18-FDG PET/CT in high grade non-Hodgkin's lymphoma

Learning Objectives Brief overview of London criteria.Need and choice of background FDG uptake.Role of quantitative and qualitative analysis.Differences in interpretation in interim PET/CT done after 2 cycles (PET/CT2) and PET/CT done after 4 cycles of chemotherapy (PET/CT4).

Baseline scan should be obtained in all the patients to establish the FDG avidity of the lymphoma and for quantitative analysis.Current evidence says that liver background should be taken as reference background and positive PET is defined when a hottest lesion has a SUVmax of 25% greater than the liver background.To overcom interobserver and inter centre differences London criteria have been proposed by Micheal Meignan et al which score the lesions from 1-5 depending on the intensity of uptake relative to tissue background, mediastinal blood pool and liver background (Fig).A score of X is given to a site of new FDG uptake which is unlikely to be lymphomatous involvement. In our experience scores(greater than liver) of 4 and 5 in PET/CT4 were associated with progression in 86% of the patients.Current evidence says that quantitative analysis performs better than visual analysis in interpreting PET/CT2. However in interpreting PET/CT4 both qualitative and quantitative analyses appear to perform equally. As elucidated by Micheal Meignan et al PET/CT findings in PET/CT2 and PET/CT4 represent different tumour biologies. PET/CT2 findings represent the chemosensitivity of tumour cells and quantitation is needed to determine how chemosensitive the tumour is ,whereas positive PET/CT4 usually represents regrowth of resistant clone of cells which are unlikely to respond to further cycles of chemotherapy and may have more potential for progression of disease.In our experience in a prospective study of 14 patients 3/7 patients with positive PET/CT2 became negative in PET/CT4 and did not progress in one year, whereas rest of the 4/7 patients with positive PET/CT4 progressed within 8 months of the study period